The overall aim of this project is to develop an integrated system of computer programs for macromolecular structure determination by X-ray crystallography. The system is designed to be highly flexible and efficient, making it suitable for application to a variety of projects in many different laboratories. The system has a general control language, a standardized data file format, a set of nucleus routines for input/output, and a collection of about a dozen major crystallographic routines. These include routines for film and diffractometer data reduction, multiple isomorphous replacement phasing, Fourier synthesis, and others. We are developing new computational techniques for refinement of atomic positions in macromolecules and for local scaling and empirical absorption corrections based on the use of Fourier-Legendre series expansions. The system is being developed in parallel for large and small computers. Extensive documentation is provided and the system is being made available to all interested crystallographers.